This invention relates to a method for removing acidic contaminants from organic extractants. More specifically, this invention relates to a method of removing acidic contaminants resulting from the preparation and from the radiolytic and hydrolytic degradation of neutral mono- and bifunctional organophosphorous compounds.
The recovery of multivalent lanthanide and actinide values from nitric acid nuclear fuel waste reprocessing solutions containing these values in combination with other metal and fission product values using the the alkyl(phenyl)-N,N-dialkylcarbamoylmethylphosphine oxides, hereinafter refered to as CMPO, is described in U.S. Pat. No. 4,548,790, dated Oct. 22, 1985. The combination of the CMPO extractants with a phase modifier such as tri-n-butylphosphate (hereinafter referred to as TBP) in a normal paraffin hydrocarbon (NPH) diluent is described in U.S. Pat. No. 4,574,072, dated Mar. 4, 1986. Both patents are assigned to the common assignee and are incorporated herein by reference.
The neutral organophosphorous compounds used for the reprocessing of irradiated nuclear reactor fuels, including monofunctional compounds such as TBP and bifunctional compounds such as the CMPO's, described above, and similar compounds such as dihexyl N,N-diethyl-carbamoylmethylenephosphonate (DHDECMP) are subject to a gradual, but continous hydrolytic and radiolytic degradation process which produce acidic compounds. These acidic compounds act as extractants, destroying the effectiveness of the extractants by preventing the effective stripping of certain radioactive values from the extractant phase. This problem is described in detail in Solvent Extraction and Ion Exchange 4(4), 677-723 (1986).
Furthermore, the preparation of these extractants produces soluble organic acidic compounds which also act as extractants. As explained above, the presence of these acidic extractants prevent stripping from taking place, greatly reducing the efficiency of the extractants and eventually reducing the ability to effect separation of the various values. This, in turn, results in the transuranium elements (TRU) either being recycled via the organic phase back into the extraction stages or the generation of a new transuranium element waste in the solvent clean-up reagent. The presence of TRU's in recycled organic solvent significantly reduce the decontamination factor than can be achieved using these extractants.
Several methods have been described for the purification of various neutral organophosphorous extractants. For example, U.S. Pat. No. 3,708,508 describes the purification of tri-n-butylphosphate (TBP) of chemical and radiolytic degradation products and fission products by passing the TBP through a strong base macroreticular anion exchange resin which absorbs many of the degradation products and metal values while recovering the purified TBP as the eluent.
Another method for purifying bidentate organophosphorous compounds of an unknown acidic contaminents is provided by U.S. Pat. No. 4,051,203. As described therein, the organophosphorous compounds, such as DHDECMP is diluted with a water-immiscible organic solvent such as CCl.sub.4, to form an extractant. The extractant is then contacted with ethylene glycol, whereby the impurities are taken up by the ethylene glycol which is then separated from the purified extractant.
However, none of these processes are completely satifactory for purifying the neutral organophosphorous extractants and furthermore, none of the processes will purify a mixture of a bifunctional extractant such as CMPO and a monofunctional extractant such as TBP.